Buick didnt make a 307. If its a buick V8, then the distributor would be at the front of the motor. The Olds engine will have the distributor at the rear.
Olds heads are marked in the spot you are describing, but would not use an X, maybe an A or a. If its an Olds motor, there will be a shelf right behind the water pump, but infront of the intake, that will have cast in numbers that will help you figure out what Olds motor it is. Also, later model Olds(1977 and up) will have the size cast into the side of the block.